#include <Eigen/Core>
#include <iostream>
using namespace Eigen;
using namespace std;

template<typename Derived>
Eigen::VectorBlock<Derived, 2>
firstTwo(MatrixBase<Derived>& v)
{
	return Eigen::VectorBlock<Derived, 2>(v.derived(), 0);
}

template<typename Derived>
const Eigen::VectorBlock<const Derived, 2>
firstTwo(const MatrixBase<Derived>& v)
{
	return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0);
}

int
main(int, char**)
{
	Matrix<int, 1, 6> v;
	v << 1, 2, 3, 4, 5, 6;
	cout << firstTwo(4 * v) << endl; // calls the const version
	firstTwo(v) *= 2;				 // calls the non-const version
	cout << "Now the vector v is:" << endl << v << endl;
	return 0;
}
